The development of wireless communication systems has increased the demand for monolithically integrated, low-cost and low-phase-noise voltage controlled oscillators (VCO:s). At the same time, the development for several communication standards utilizing different frequency bands has pushed researchers to develop multi-band as well as multi-standard transceivers. This has, in turn, forced researchers to look for multi-band VCO:s, especially dual-band VCO:s. One of the major driving forces for the development of such transceivers is the need for multi-band VCO:s with good phase noise performance.
In brief, a voltage-controlled oscillator (VCO) is a fundamental building block in a transceiver system. It provides a local oscillation (LO) signal source for a mixer that translates an intermediate frequency (IF) to a radio frequency (RF), or vice versa. For a dual-band transceiver system, two single-band VCO:s or one dual-band oscillator is utilized.
There are several known approaches to design and build a dual-band VCO. For instance, switched capacitors or inductors can be used to construct a dual-band VCO, as shown in FIG. 1 [1], [2]. Unfortunately, the switch's parasitic capacitance and channel resistance have negative impact on the VCO tuning range and phase noise performance. Recently, the mutual inductances of two coupled transmission lines or a transformer have been utilized to build a dual-band VCO [3], as shown in FIG. 2 and FIG. 3. For further information concerning the concept of mutual inductance, see Appendix 1. In such a VCO, the frequency band is set and determined by the phase difference between the respective currents I1 and I2, as shown in FIG. 3(c). When the two currents I1 and I2 are in-phase the total inductance is determined according to Li=Ls,i+M, and when I1 and I2 are in anti-phase the total inductance is determined according to Li=Ls,j−M, where Li (i=1,2) is the total inductance of a primary or secondary coil, Ls,j is the self-inductance, M is mutual inductance. The corresponding oscillation frequency of the oscillator device then becomes, according to Equation 1:
                              f          0                =                  1                      2            ⁢            π            ⁢                                                            (                                                            L                                              s                        ,                        j                                                              ±                    M                                    )                                ⁢                C                                                                        (        1        )            
Here, for the sake of simplicity, the respective inductance Lp1 and Ls1 are assumed to have the same value Ls1, and the capacitances Cp1 and Cs1 to have the same value C. In practice, the aforementioned transformer together with parallel capacitors, as shown in FIG. 2(b) and FIG. 3(c), make up a dual-mode resonator, and its frequency band depends on the phase difference of I1 and I2.
Even though using transformers in dual-band VCO's avoids the problems caused by switches in the resonator, a transformer has its own problems, i.e. its quality factor becomes poor when the mutual inductance is negative [3]. The relation between the resonance frequency and resonance width of the oscillator typically determines the quality factor.
Therefore, there is a need for a dual-band voltage-controlled oscillator arrangement that avoids or reduces the above-mentioned disadvantages.